1. Field of the Invention
The present invention relates to a shunt regulator for regulating programming voltage employed to erase or write to nonvolatile memory devices, and more particularly concerns regulation by means of components that are subject to manufacturing process variations and wear out mechanisms that are the same as manufacturing process variations and wear out mechanisms of the regulated devices.
2. Description of Related Art
Programming voltages employed to provide power to various nonvolatile programmable devices during device programming are often generated at a level above the level actually required in order to ensure that the voltages actually applied will be adequate. For example, if a programming voltage of 10 volts is required for operation of an ideal circuit, it is possible that variations in manufacturing processes could cause the circuit to require a voltage of several volts more than the nominal. Accordingly, voltage sources are established for the circuit to account for a higher voltage that could be required, and voltage regulators are employed to limit the higher voltage generated to the desired value.
Shunt voltage regulators of the prior art have been based primarily on zener diodes or band gap references, which have several disadvantages. In many integrated circuit manufacturing processes zener diodes cannot be manufactured without adding additional processing steps. The use of band gap reference for voltage regulation requires a large area of the chip, and, furthermore, draws DC current. Neither zener diode nor band gap based regulators will track manufacturing process variations or the charge trapping wear out of the nonvolatile programmable devices to be regulated.
In the manufacture of integrated circuit devices operation of the components and voltages that are required at various points in the circuit depend on a number of different process steps. Various layers of material are deposited with ideally predetermined precision and precisely controlled thicknesses. However, should the actual thickness of some layers vary, or should one or more of the deposited layers have other than the predetermined impurities therein, circuit operation and circuit voltages of the resulting product will vary. Prior shunt regulators, such as zener diodes and band gap based regulators, establish a predetermined fixed regulation and will not track process manufacturing process variations. That is, they will not vary the level of regulated voltage as may be required by variations in manufacturing processes. It has been established that floating gate devices exhibit a wear out mechanism caused by trapped charge in the tunneling oxide. This manifests itself as reduced threshold shifts during the program cycle.
Regulators based on MOSFET threshold voltage are exceedingly sensitive to process and temperature variations and, moreover, will not track the manufacturing process of nonvolatile circuits.
Accordingly, it is an object of the present invention to provide a voltage regulator that avoids or minimizes above mentioned problems.